The present invention relates to a pretensioner which engageably holds a tongue of a seat belt and also automatically tenses the seat belt with the tongue being engaged and held according to detection of a predetermined trigger.
Generally, a seat belt apparatus for restraining an occupant is provided in order to prevent the occupant from being thrown forward due to an inertial force created by an impact such as a vehicle collision.
Also, such a seat belt apparatus is generally provided with an Emergency Locking Retractor (hereinafter, referred to as an ELR) which does not restrain motion of the occupant in normal driving but prevents the seat belt from being further withdrawn by locking a retraction shaft of the seat belt only when the occupant is subjected to an impact, for example, when the vehicle suddenly stops or is involved in a collision.
The ELR locks the retraction shaft within a very short time when detecting a deceleration equal to or higher than a predetermined value, so that the seat belt will not be withdrawn further.
However, according to the above-described ELR which merely locks the retraction shaft, in a case where the notwithdrawn seat belt is loosely retracted around the retraction shaft while restraining the occupant, a certain amount of the seat belt may be withdrawn even when the retraction shaft is locked. Also, even when the seat belt is not loosely retracted, the seat belt may still be withdrawn from the time when the deceleration equal to or higher than the predetermined value is detected up to the time when the retraction shaft is locked.
The looseness of the seat belt and the withdrawal of the seat belt caused by the time lag up to the time when the retraction shaft is locked should be minimized.
There is a known pretensioner which draws in a seat belt buckle when the same deceleration as that for a locking operation of the retraction shaft is detected and which serves as a device for canceling, in some measure, the looseness of the seat belt and the withdrawal of the seat belt caused by the time lag up to the time when the retraction shaft is locked such as, for example, shown in Japanese Domestic Re-Publication No. 2003-018374 (FIGS. 2 and 5) of PCT International Application (“JP Pub. No. '374”) (which is incorporated by reference herein in its entirety).
FIG. 7 is an explanatory illustration of a vehicle seat and the peripheral portion showing an example of a seat belt apparatus provided with such a pretensioner.
In FIG. 7, a seat belt 3 is withdrawn from an ELR 2, which is arranged on a lower portion of a center pillar 1 on an inner side of a vehicle cabin, to an upper side of a vehicle body, passes through a through-ring 4 attached on an upper portion of the center pillar 1 and then extends to a lower side of the vehicle body, so that an end 3a of the seat belt 3 is fixed on an inner wall of the vehicle body between the center pillar 1 and a seat 5.
A tongue 6 is provided movably along the seat belt 3 at a portion of the seat belt 3 between the through-ring 4 and the end 3a. 
The tongue 6 is held by a seat belt buckle 7, which protrudes upward from the seat 5 on an inner side of the vehicle body, by engaging with the seat belt buckle 7.
The seat belt buckle 7 is held on an upper end of a wire 8. Additionally, the seat belt buckle 7 and the wire 8 define a part of a pretensioner 9.
FIGS. 8 and 9 each show an example of the pretensioner 9, FIG. 8 showing a cross section of the pretensioner and FIG. 9 showing an exploded perspective view of the pretensioner.
The pretensioner 9 includes a base plate 10 fixed on a lateral surface of a sitting portion of the seat 5; a cover 11 (not shown in FIG. 8) holding an end of the wire 8 in cooperation with the base plate 10; a retainer 12, a washer 13, a screw 14 and a washer 15 for combination coupling of the base plate 10 and the cover 11; a guide pin 16 guided in long holes 10a and 11a which are respectively formed in the base plate 10 and the cover 11 and extend in a front-to-back direction of the vehicle; a piston rod 17 engaged with the guide pin 16; and a gas generator 18 for pushing out the piston rod 17.
Note that the gas generator 18 may be any known device, such as an actuator, a solenoid, an inflator or the like, which extends the piston rod 17 in response to a detection result, as a trigger, of a detector (not shown) which detects a deceleration of the vehicle.
A tip end 17a of the piston rod 17 forms a bent portion by bending an intermediate portion of the wire 8. The wire 8 extends in a vertical direction of the vehicle and in the front-to-back direction of the vehicle, and the bent portion serves as the boundary. Also, the guide pin 16 penetrates through the tip end 17a, so that the piston rod 17 extends along the long holes 10a and 11a. 
The seat belt buckle 7 is fixed to an upper end of the wire 8, while a lower end of the wire 8 is fixed by the base plate 10 and the cover 11 in a fixed position. In addition, the intermediate portion of the wire 8 penetrates through the retainer 12.
With this configuration, since the piston rod 17 extends when the gas generator 18 is actuated, the upper end of the wire 8 is displaced in a draw-in direction integrally with the seat belt buckle 7, thereby canceling the withdrawal of the seat belt 3 from the ELR 2.
A cut raised portion 11b is provided in the cover 11. The cut raised portion 11b is formed by cutting and raising a part of the cover 11 inwardly so as to abut on an outer periphery of a curved portion of the wire 8 in a vicinity of the intermediate bent profile portion of the wire 8. The bent profile portion is biased by the piston rod 17. Accordingly, in normal use, a displacement of the wire 8 is restrained by the cut raised portion 11b even when the wire 8 is pushed in by the tongue 6 of the seat belt 3 pushed into the seat belt buckle 7 for fastening.
When the piston rod 17 extends upon the actuation of the gas generator 18 and the bent profile portion of the wire 8 is externally deformed and displaced, the cut raised portion 11b provides a fuse function by which the cut raised portion 11b is restored from the cut and raised state freely.